Cholesterol esters, synthesized from cholesterol and fatty acyl CoAs by acyl CoA:cholesterol acyltransferase (ACAT) enzymes, are neutral lipids that play important roles in cholesterol absorption, the synthesis and secretion of apolipoprotein (apo) B-containing lipoproteins, and the formation of macrophage foam cells in atherosclerotic lesions. In recent years, our understanding of the biology of ACAT enzymes has increased significantly, and work from this grant, including the identification of ACAT2 and the generation of mutant mice lacking ACAT1 and ACAT2, has contributed significantly to these advances. However, many important questions remain unanswered. The current proposal is organized around specific aims that address some of the most important of these questions. In the first aim, we will use ACAT2-/- mice to investigate the relative atherogenicity of apoB-containing lipoproteins that contain primarily triglycerides, rather than cholesterol esters, in their cores. These experiments will investigate whether triglyceride-rich lipoproteins promote atherosclerosis. The second aim is to determine the relationship of ACAT2 expression in the intestine and liver to cholesterol absorption and cholesterol gallstone formation. ACAT2-/- mice will be used to further define the role of ACAT2 in intestinal cholesterol absorption and hepatic very low-density lipoprotein secretion. We will also test the hypothesis that ACAT2 deficiency in the intestine is protective against gallstone formation, whereas selective ACAT2 deficiency in the liver promotes gallstone formation. The third aim is to determine whether cholesterol ester synthesis by ACAT1 and ACAT2 are coupled with distinct cellular functions. Experiments utilizing adenovirus-mediated gene transfer and gene "knock-ins" are proposed to determine if both ACAT1 and ACAT2 can synthesize cholesterol esters for secretion in apoB-containing lipoproteins. The fourth aim is to determine the function of ACAT enzymes in embryonic development by studying ACAT gene expression during development and investigating the cause of lethality of ACAT1-/-ACAT2-/- embryos.